In the operation of an internal combustion engine, it is known to interrupt the supply of fuel when the throttle valve is closed at high engine speeds, that is, when the internal combustion engine is in the overrun mode of operation. However, an overrun condition exists also if the engine speed of an internal combustion engine is higher than that corresponding to the throttle position in a spark-ignition engine or to the quantity of fuel injected in a Diesel engine. If the internal combustion engine is in overrun operation, then no output is desired. Therefore, fuel delivery to the internal combustion engine through carburetors, injection systems or the like is reduced or interrupted entirely.
In this manner, substantial fuel savings can be realized; on the other hand, the overrun mode is not without problems since the interruption of the fuel supply causes the internal combustion engine to cool down a certain amount, followed by higher pollutant emissions for a period of time following termination of the overrun mode of operation; and, it may also adversely affect the driving comfort when the engine switches from the overrun to the normal mode.
Another problem is that the engine speed behavior always has to be monitored closely, that is, the engine is not permitted to stall, not even if the supply of fuel is interrupted during overrunning while the engine is still cold. Thus, for example, a critical load may be encountered when, with the overrun cutoff mode enabled, the vehicle drives down hill with the engine still cold, that is, when the fuel supply is interrupted while the throttle valve is closed, and when the clutch is suddenly disengaged so that the internal combustion engine is no longer driven by the rotational movement of the wheels through the transmission. In such a case, there is the danger of the engine speed dropping so abruptly that the engine stalls before countermeasures can be taken.
Therefore, in a system providing for the interruption of the fuel supply with the engine overrunning, as disclosed in copending U.S. patent application entitled "Method for Operating an Apparatus for a Fuel Control System of an Internal Combustion Engine during Overrunning", having Ser. No. 410,669 and filed on Aug. 23, 1982, it is known to compare the actual engine speed with a predetermined, time-dependent characteristic of a resume speed and to interrupt the supply of fuel to the internal combustion engine only if the actual engine speed is above the characteristic of the resume speed. This makes it possible to sense the relevant operating state more accurately; the overrun cutoff mode is disabled if the engine speed drops below the desired resume-speed characteristic, and jerks caused by fuel-delivery resumptions can be avoided, thus adding to the driving comfort. Nevertheless, the known system for shutting off the fuel supply in the overrun mode of operation does not lend itself to universal use, nor is its response to all possible operating conditions sufficiently flexible. In particular, the vehicle behavior in the range of the resume speed may be problematic, with speed hunting being a possible occurrence. In addition, the vehicle may become subject to conditions under which it is desirable to inhibit the overrun cutoff function entirely.